Field
The present disclosure generally relates to radio-frequency (RF) power amplifiers (PAs).
Description of the Related Art
Multimode/multiband (MMMB) power amplifier modules (PAM) can face competition from envelope tracking (ET) PAMs in, for example, the 4G LTE market. To be successful in such a market, it is desirable for MMMB PAMs to provide significant performance improvements over existing MMMB PAMs and also to be able to compete with ET-based PAMs (e.g., based on cost, performance and size).
In the 4G LTE standard, newer modulation protocols such as OFDMA (orthogonal frequency-division multiple access) and SC-FDMA (single carrier FDMA) can be used to support high data-rate in scalable channel bandwidths of, for example, 5 MHz-20 MHz and even 40 MHz. High data rate transmission using a number of independently modulated subcarriers over a variable channel bandwidth in 4G LTE communication protocol typically comes at a cost of high peak to average power ratio (PAPR) for the transmitter. From the perspective of the power amplifier, a high PAPR modulation scheme typically translates into low average efficiency as the power amplifier typically needs to operate in deep-back-off mode most of the time, away from its peak efficiency point, to prevent clipping of transmitted signals. Accordingly, newer architecture of power amplifiers are desirable for 4G standards that can operate with high linearity and high PAE even under significant back-off conditions.
Compared to envelope tracking PAMs for efficiency enhancement under back-off, Doherty PAMs are capable of meeting high efficiency with high linearity under back-off with much reduced system complexity and reduced calibration and digital pre-distortion (DPD) specifications. However, some Doherty power amplifier architectures are bandwidth limited due to the narrowband nature of existing Doherty power combiners. Further, some Doherty PAMs can have large AM-AM (amplitude-modulation-to-amplitude-modulation) and AM-PM (amplitude-modulation -to-phase-modulation)distortion at back-off due to dynamically changing load-lines, which can affect the FOM (figure of merit) achievable at rated power levels for 4G LTE communication standards. For a broad-band Doherty PAM, being able to control such distortions at back-off conditions can be desirable, especially when implemented over wide-band.